In the use of offshore marine platforms which are fixedly positioned to the sea floor by pilings and the like, the problem of moving water and ocean currents can be troublesome in several ways. For one thing, constantly changing sea conditions can affect excessive periodic waves and water currents which could in turn set up harmful harmonics in the platform movement.
Further, in deeper offshore waters it is usually necessary to provide considerable weight at the platform lower end. This is to stabilize it more firmly against the above noted wind and wave conditions as well as to provide balance to the greater amount of steel necessitated in such deep water structures.
In the instance of any offshore platform however, a primary problem is the one caused by movement of water about the platform anchored or lower end. In virtually any location there will be a certain amount of water flow due to prevailing currents, to tide variations, and to other natural phenomena. In the instance of the firmly positioned or piled platform this water movement can be particularly harmful where the water tends to scour away the soil beneath the platform. Such action will tend to expose a greater length of the pile as well as a portion of the anchoring legs.
The stated problem has been particularly accentuated in the instance of platforms where the bottom structure is supplemented by tanks, storage facilities, templates or other sub-structures which would, by their presence, tend to alter the normal course of the flowing water. For example, where a storage facility has been incorporated into the platform lower end, water diverted through the platform legs has been found to scour a sufficient quantity of the ocean floor away from the anchoring piles and legs to jeopardize the stability of the structure.
In the arrangement of the marine structure presently contemplated, the lower end of the platform adjacent the ocean floor is provided with a relatively heavy apron or substantially horizontal skirt. A primary function of this apron, as it will be hereinafter referred to, is to engage the lower end of the platform to provide an additional weight factor thereby stabilizing the overall structure.
Since the apron is disposed adjacent to and about the respective legs of the platform, it in effect prohibits the scouring effect of ocean currents which would otherwise tend to erode away the sand and substrate and form shallow pits about the leg and anchoring piles.
The apron is formed preferably by pouring or depositing fluidized concrete or cement from a source above the water's surface, to the ocean floor. The concrete is initially retained within a flexible walled container which defines a partial enclosure about the fluidized material. The character of the flexible retainer is such that it will permit the fluidized material to normally flow about the platform legs, and yet maintain a desired depth. Thereafter, as the concrete hardens it will form a rigid yet widespread base or foundation. The flexible retainer can then be removed if feasible, or merely be permitted to wear away in the course of time such that only the concrete apron remains.